A guided wave can be generated by the passage of AC current through a coil wound around a subject, for example. That is, AC current flowing through a coil wound around a subject generates AC magnetic field. A magnetic force from this AC magnetic field can be used to vibrate the subject, whereby a guided wave as a kind of sound waves can be generated. The thus generated oscillation wave propagates through the subject along the longitudinal direction thereof.
A reflected wave of the guided wave is detected, whereby the soundness of the subject can be tested. A guided wave is reflected as a reflected wave at a discontinuous part of the subject or at a part of a change in cross-sectional area of the subject in the circumferential direction. This reflected wave is detected at an oscillation position of the guided wave, whereby the soundness of the subject is tested. The soundness of the subject is tested about the presence or absence of a defect part such as a flaw, corrosion or the like of the subject.
Examples of the guided wave include an L-mode (Longitudinal mode) guided wave and a T-mode (Torsional mode) guided wave. The L-mode guided wave propagates through a subject while vibrating in the propagation direction, and the T-mode guided wave propagates through a subject while torsionally vibrating in the subject.
Such a guided wave suffers less attenuation than a sound wave used for general sound wave testing, and therefore enables testing of a subject for the soundness over a wide range of the subject. A sound wave used for general sound wave testing has a high frequency of 5 MHz and a small wavelength of 0.6 mm, for example, and for this reason, is likely to attenuate. On the other hand, the above-stated guided wave has a small frequency of 32 kHz and a large wavelength of 100 mm, for example, and for this reason, suffers less attenuation.
The following Patent Document 1 is available, for example, as a prior art document of the present application.